In recent years, the use of electrostatic chucks in the semiconductor industry has increased because these chucks exhibit excellent characteristics in a vacuum. In addition, electrostatic chucks have several advantages over mechanical and vacuum chucks. For example, electrostatic chucks reduce stress-induced cracks caused by mechanical clamps, allow processing of a larger portion of the wafer, and can be used in processes conducted at low pressure.
A typical monopolar electrostatic chuck is made up of an electrode covered by a dielectric. When the electrode is electrically charged, an opposing electrostatic charge accumulates in the wafer and the resultant electrostatic force holds the wafer on to the electrostatic chuck.
The original concept for a monopolar electrostatic chuck was described by G. Wardly, “Electrostatic Wafer Chuck for Electron Beam Microfabrication,” Rev. Sci. Instr. 44, 1506 (1973). A wafer, acting as one plate of a capacitor, is placed on a metal chuck covered with a dielectric that acts as the other plate of the capacitor. It is well known there is a strong attractive force between the two plates of a capacitor when they are held at different potentials. Consequently, the wafer is attracted to the metal chuck when the wafer is held at one electrical potential while the metal chuck is held at another electric potential. Because the metal chuck is only charged at one potential, this arrangement is referred to as a monopolar electrostatic chuck.
Further developments in monopolar technology have subsequently been described in various issued patents. For example, Livesay (U.S. Pat. No. 3,983,401, Sep. 28, 1976) and McGinty (U.S. Pat. No. 3,993,509, Nov. 23, 1976) describe a monopolar electrostatic chuck configured such that the wafer is supported on a flat pedestal or supported vertically in a wedge configuration, respectively.
However, problems arise in the monopolar configuration because a voltage must be applied to the wafer in order to produce the desired attractive chucking force. This limits the wafers to be held to conductors or semiconductors or at least to be coated with a conducting layer. Consequently, a silicon wafer coated with a layer of oxide (SiO2) cannot be held by a monopolar chuck.
Some of the problems associated with non-conducting wafers were addressed in later patents. For example, Wachtler (U.S. Pat. No. 3,916,270, Oct. 28, 1975), Briglia (U.S. Pat. No. 4,184,188, Jan. 15, 1980) and Wicker (U.S. Pat. No. 4,724,510, Feb. 9, 1988) recognized that it was not necessary to contact the wafer if a split electrode concept was used. That is, when the lower electrode of a capacitor is split into two equal parts and separated by an insulator, with each half placed at equal but opposite voltages (e.g. +V and −V), then the upper electrode (the wafer) must be at ground potential (V=0) because of symmetry. In this case, there is an absolute value potential difference of ‘V’ between each half of the lower electrode and its respective portion of the upper electrode (the wafer). The attractive force between two plates of a capacitor depends on the voltage difference squared, so there are equal holding forces on each wafer half. Most importantly, it is not necessary to make contact with the wafer surface in order to maintain the preferred wafer voltage (preferably zero). Because the lower electrode of the capacitor is split into two equal parts with each half placed at equal but opposite voltages, this arrangement is referred to as a bipolar electrostatic chuck.
However, Wachtler, Briglia and Wicker's electrode geometries are fairly difficult to fabricate. Abe (U.S. Pat. No. 4,384,918 May 24, 1983) described a simpler arrangement wherein each electrode is a simple half circle. Nevertheless, fabricating such a bipolar chuck presents many practical difficulties. Such difficulties arise because (1), the force of attraction is very sensitive to flatness of the chuck surface and (2), it is difficult to electrically isolate each half electrode.
A further refinement on the bipolar concept was described in Suzuki (U.S. Pat. No. 4,692,836, Sep. 8, 1987). Suzuki pointed out that by using a radially segmented bipolar design, a wafer that is initially bowed up in the center can be more easily flattened by activating the central electrodes first. Unfortunately, practical semiconductor wafers are more likely to be warped rather than simply bowed. Therefore, Suzuki design offers no solution to this problem.
Wafer retention to the chuck is another issue for electrostatic chucks. When a DC voltage is applied, the dielectric separating the wafer and the metal chuck can become permanently polarized and, after the voltage is removed, the residual polarization can hold the wafer to the chuck for some time. Horwitz (U.S. Pat. No. 5,103,367, Apr. 7, 1992) suggested a solution to the problem of wafer retention by describing an AC chuck. Horowitz suggested the use of sapphire or boron nitride as the dielectric material because of the ability of such materials to transfer RF power efficiently. However, Horowitz did not describe any method of fabricating such a chuck. In particular, when two pieces of a single crystal material are joined by a high temperature process, it is important to know whether or not the crystalline material is anisotropic. In other words, when heated, it may expand different amounts in different crystal orientations. In such cases, when the joined parts cool to room temperature, the assembly warps. It should be noted that large diameter discs or wafers of sapphire or boron nitride, as required by the Horowitz technique, are prohibitively expensive. At the same time, AC excitation of an electrostatic chuck introduces many practical difficulties in designing and operating such a system.
Another approach to dealing with the problem of slow wafer release is described in Watanabe (U.S. Pat. No. 5,117,121, May 26, 1992). Watanabe describes a chuck made of ceramic that is inherently susceptible to retention forces. He proposed effecting the release of the wafer by applying a high reverse bias voltage (1.5-2 times). However, this high voltage increases the risk of breakdown of the dielectric and is difficult to control in a practical circuit. For example, if the reverse bias is held too long, the wafer will stick again and not release.
Another issue inherent to current monopolar and bipolar electrostatic chuck designs is particle contamination on the backside of the wafer. The level of backside particulate contamination is a function of the contact area between the wafer and electrostatic chuck, surface finish of the electrostatic chuck contact area, and mechanical stress in the contact points. Particle contamination on the backside of wafers has become a serious issue in advanced microelectronics manufacturing for several reasons. One reason is particles on the backside of the wafer can cause cross-contamination and electrical contact failures in interconnect structures. A second reason is the change in wafer planarity associated with such contamination. Specifically, particles present on the backside of the wafer can impact control over the critical dimension in lithographic processes by causing wafer warpage.
Accordingly, there is a need for a practical, reliable, and less expensive electrostatic chuck for holding and reliably releasing wafers while minimizing backside wafer contamination by limiting wafer contact with the electrostatic chuck.